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                                <p>&#x8FD9;&#x7BC7;&#x6559;&#x7A0B;&#xFF0C;&#x6211;&#x4EEC;&#x5C06;&#x5B66;&#x4E60; Elixir &#x8BED;&#x8A00;&#x4E2D;&#x7684;&#x63A7;&#x5236;&#x8BED;&#x53E5;&#x3002;</p>
<h2 id="if-&#x548C;-unless"><code>if</code> &#x548C; <code>unless</code></h2>
<p>&#x4F60;&#x4E4B;&#x524D;&#x53EF;&#x80FD;&#x9047;&#x5230;&#x8FC7; <code>if/2</code>&#x4E86;&#xFF0C;&#x5982;&#x679C;&#x4F60;&#x4F7F;&#x7528;&#x8FC7; Ruby&#xFF0C;&#x4E5F;&#x4F1A;&#x5F88;&#x719F;&#x6089; <code>unless/2</code>&#x3002;&#x5B83;&#x4EEC;&#x5728; Elixir &#x4F7F;&#x7528;&#x65B9;&#x5F0F;&#x4E5F;&#x4E00;&#x6837;&#xFF0C;&#x53EA;&#x4E0D;&#x8FC7;&#x5B83;&#x4EEC;&#x5728; Elixir &#x91CC;&#x662F;&#x5B8F;&#x5B9A;&#x4E49;&#xFF0C;&#x4E0D;&#x662F;&#x8BED;&#x8A00;&#x672C;&#x8EAB;&#x7684;&#x8BED;&#x53E5;&#x3002;&#x4F60;&#x53EF;&#x4EE5;&#x5728; <a href="https://hexdocs.pm/elixir/Kernel.html" target="_blank">Kernel &#x6A21;&#x5757;</a> &#x627E;&#x5230;&#x5B83;&#x4EEC;&#x7684;&#x5B9E;&#x73B0;&#x3002;</p>
<p>&#x9700;&#x8981;&#x6CE8;&#x610F;&#x7684;&#x662F;&#xFF0C;Elixir &#x4E2D;&#x552F;&#x4E00;&#x4E3A;&#x5047;&#x7684;&#x503C;&#x662F; <code>nil</code> &#x548C; &#x5E03;&#x5C14;&#x503C; <code>false</code>&#x3002;</p>
<pre><code class="lang-elixir">iex&gt; if String.valid?(<span class="hljs-string">&quot;Hello&quot;</span>) <span class="hljs-keyword">do</span>
...&gt;   <span class="hljs-string">&quot;Valid string!&quot;</span>
...&gt; else
...&gt;   <span class="hljs-string">&quot;Invalid string.&quot;</span>
...&gt; <span class="hljs-keyword">end</span>
<span class="hljs-string">&quot;Valid string!&quot;</span>

iex&gt; if <span class="hljs-string">&quot;a string value&quot;</span> <span class="hljs-keyword">do</span>
...&gt;   <span class="hljs-string">&quot;Truthy&quot;</span>
...&gt; <span class="hljs-keyword">end</span>
<span class="hljs-string">&quot;Truthy&quot;</span>
</code></pre>
<p><code>unless/2</code> &#x4F7F;&#x7528;&#x65B9;&#x6CD5;&#x548C; <code>if/2</code> &#x4E00;&#x6837;&#xFF0C;&#x4E0D;&#x8FC7;&#x53EA;&#x6709;&#x5F53;&#x5224;&#x65AD;&#x4E3A;&#x5426;&#x624D;&#x4F1A;&#x7EE7;&#x7EED;&#x6267;&#x884C;&#xFF1A;</p>
<pre><code class="lang-elixir">iex&gt; <span class="hljs-keyword">unless</span> is_integer(<span class="hljs-string">&quot;hello&quot;</span>) <span class="hljs-keyword">do</span>
...&gt;   <span class="hljs-string">&quot;Not an Int&quot;</span>
...&gt; <span class="hljs-keyword">end</span>
<span class="hljs-string">&quot;Not an Int&quot;</span>
</code></pre>
<h2 id="case"><code>case</code></h2>
<p>&#x5982;&#x679C;&#x9700;&#x8981;&#x5339;&#x914D;&#x591A;&#x4E2A;&#x6A21;&#x5F0F;&#xFF0C;&#x6211;&#x4EEC;&#x53EF;&#x4F7F;&#x7528; <code>case</code>&#xFF1A;</p>
<pre><code class="lang-elixir">iex&gt; <span class="hljs-keyword">case</span> {<span class="hljs-symbol">:ok</span>, <span class="hljs-string">&quot;Hello World&quot;</span>} <span class="hljs-keyword">do</span>
...&gt;   {<span class="hljs-symbol">:ok</span>, result} -&gt; result
...&gt;   {<span class="hljs-symbol">:error</span>} -&gt; <span class="hljs-string">&quot;Uh oh!&quot;</span>
...&gt;   <span class="hljs-number">_</span> -&gt; <span class="hljs-string">&quot;Catch all&quot;</span>
...&gt; <span class="hljs-keyword">end</span>
<span class="hljs-string">&quot;Hello World&quot;</span>
</code></pre>
<p><code>_</code> &#x53D8;&#x91CF;&#x662F; <code>case/</code> &#x8BED;&#x53E5;&#x91CD;&#x8981;&#x7684;&#x4E00;&#x9879;&#xFF0C;&#x5982;&#x679C;&#x6CA1;&#x6709; <code>_</code>&#xFF0C;&#x6240;&#x6709;&#x6A21;&#x5F0F;&#x90FD;&#x65E0;&#x6CD5;&#x5339;&#x914D;&#x7684;&#x65F6;&#x5019;&#x4F1A;&#x629B;&#x51FA;&#x5F02;&#x5E38;&#xFF1A;</p>
<pre><code class="lang-elixir">iex&gt; <span class="hljs-keyword">case</span> <span class="hljs-symbol">:even</span> <span class="hljs-keyword">do</span>
...&gt;   <span class="hljs-symbol">:odd</span> -&gt; <span class="hljs-string">&quot;Odd&quot;</span>
...&gt; <span class="hljs-keyword">end</span>
** (CaseClauseError) no <span class="hljs-keyword">case</span> clause <span class="hljs-symbol">matching:</span> <span class="hljs-symbol">:even</span>

iex&gt; <span class="hljs-keyword">case</span> <span class="hljs-symbol">:even</span> <span class="hljs-keyword">do</span>
...&gt;   <span class="hljs-symbol">:odd</span> -&gt; <span class="hljs-string">&quot;Odd&quot;</span>
...&gt;   <span class="hljs-number">_</span> -&gt; <span class="hljs-string">&quot;Not Odd&quot;</span>
...&gt; <span class="hljs-keyword">end</span>
<span class="hljs-string">&quot;Not Odd&quot;</span>
</code></pre>
<p>&#x53EF;&#x4EE5;&#x628A; <code>_</code> &#x60F3;&#x8C61;&#x6210;&#x6700;&#x540E;&#x7684; <code>else</code>&#xFF0C;&#x4F1A;&#x5339;&#x914D;&#x4EFB;&#x4F55;&#x4E1C;&#x897F;&#x3002;</p>
<p>&#x56E0;&#x4E3A; <code>case/2</code> &#x4F9D;&#x8D56;&#x6A21;&#x5F0F;&#x5339;&#x914D;&#xFF0C;&#x6240;&#x4EE5;&#x4E4B;&#x524D;&#x6240;&#x6709;&#x7684;&#x6709;&#x5173;&#x6A21;&#x5F0F;&#x5339;&#x914D;&#x7684;&#x89C4;&#x5219;&#x548C;&#x9650;&#x5236;&#x5728;&#x8FD9;&#x91CC;&#x90FD;&#x9002;&#x7528;&#x3002;
&#x5982;&#x679C;&#x4F60;&#x60F3;&#x5339;&#x914D;&#x5DF2;&#x7ECF;&#x5B9A;&#x4E49;&#x7684;&#x53D8;&#x91CF;&#xFF0C;&#x4E00;&#x5B9A;&#x8981;&#x4F7F;&#x7528; pin &#x64CD;&#x4F5C;&#x7B26; <code>^/1</code>&#xFF1A;</p>
<pre><code class="lang-elixir">iex&gt; pie = <span class="hljs-number">3.14</span>
 <span class="hljs-number">3.14</span>
iex&gt; <span class="hljs-keyword">case</span> <span class="hljs-string">&quot;cherry pie&quot;</span> <span class="hljs-keyword">do</span>
...&gt;   ^pie -&gt; <span class="hljs-string">&quot;Not so tasty&quot;</span>
...&gt;   pie -&gt; <span class="hljs-string">&quot;I bet <span class="hljs-subst">#{pie}</span> is tasty&quot;</span>
...&gt; <span class="hljs-keyword">end</span>
<span class="hljs-string">&quot;I bet cherry pie is tasty&quot;</span>
</code></pre>
<p><code>case/2</code> &#x8FD8;&#x6709;&#x4E00;&#x4E2A;&#x5F88;&#x9177;&#x7684;&#x7279;&#x6027;&#xFF1A;&#x5B83;&#x652F;&#x6301;&#x536B;&#x5175;&#x8868;&#x8FBE;&#x5F0F;&#xFF08;Guard Clause&#xFF09;&#xFF1A;</p>
<p><em>&#x8FD9;&#x4E2A;&#x4F8B;&#x5B50;&#x76F4;&#x63A5;&#x53D6;&#x81EA; Elixir &#x5B98;&#x65B9;&#x6307;&#x5357;&#x7684;<a href="https://elixir-lang.org/getting-started/case-cond-and-if.html#case" target="_blank">&#x4E0A;&#x624B;&#x6559;&#x7A0B;</a></em></p>
<pre><code class="lang-elixir">iex&gt; <span class="hljs-keyword">case</span> {<span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>} <span class="hljs-keyword">do</span>
...&gt;   {<span class="hljs-number">1</span>, x, <span class="hljs-number">3</span>} <span class="hljs-keyword">when</span> x &gt; <span class="hljs-number">0</span> -&gt;
...&gt;     <span class="hljs-string">&quot;Will match&quot;</span>
...&gt;   <span class="hljs-number">_</span> -&gt;
...&gt;     <span class="hljs-string">&quot;Won&apos;t match&quot;</span>
...&gt; <span class="hljs-keyword">end</span>
<span class="hljs-string">&quot;Will match&quot;</span>
</code></pre>
<p>&#x53C2;&#x8003;&#x5B98;&#x65B9;&#x7684;&#x6587;&#x6863;&#x6765;&#x770B;<a href="https://hexdocs.pm/elixir/guards.html#list-of-allowed-expressions" target="_blank">&#x536B;&#x5175;&#x652F;&#x6301;&#x7684;&#x8868;&#x8FBE;&#x5F0F;</a></p>
<h2 id="cond"><code>cond</code></h2>
<p>&#x5F53;&#x6211;&#x4EEC;&#x9700;&#x8981;&#x5339;&#x914D;&#x6761;&#x4EF6;&#x800C;&#x4E0D;&#x662F;&#x503C;&#x7684;&#x65F6;&#x5019;&#xFF0C;&#x53EF;&#x4EE5;&#x4F7F;&#x7528; <code>cond/1</code>&#xFF0C;&#x8FD9;&#x548C;&#x5176;&#x4ED6;&#x8BED;&#x8A00;&#x7684; <code>else if</code> &#x6216;&#x8005; <code>elsif</code> &#x76F8;&#x4F3C;&#xFF1A;</p>
<p><em>&#x8FD9;&#x4E2A;&#x4F8B;&#x5B50;&#x76F4;&#x63A5;&#x53D6;&#x81EA; Elixir &#x5B98;&#x65B9;&#x6559;&#x7A0B;&#x7684; <a href="https://elixir-lang.org/getting-started/case-cond-and-if.html#cond" target="_blank">&#x4E0A;&#x624B;&#x6307;&#x5357;</a></em></p>
<pre><code class="lang-elixir">iex&gt; <span class="hljs-keyword">cond</span> <span class="hljs-keyword">do</span>
...&gt;   <span class="hljs-number">2</span> + <span class="hljs-number">2</span> == <span class="hljs-number">5</span> -&gt;
...&gt;     <span class="hljs-string">&quot;This will not be true&quot;</span>
...&gt;   <span class="hljs-number">2</span> * <span class="hljs-number">2</span> == <span class="hljs-number">3</span> -&gt;
...&gt;     <span class="hljs-string">&quot;Nor this&quot;</span>
...&gt;   <span class="hljs-number">1</span> + <span class="hljs-number">1</span> == <span class="hljs-number">2</span> -&gt;
...&gt;     <span class="hljs-string">&quot;But this will&quot;</span>
...&gt; <span class="hljs-keyword">end</span>
<span class="hljs-string">&quot;But this will&quot;</span>
</code></pre>
<p>&#x548C; <code>case/2</code> &#x4E00;&#x6837;&#xFF0C;<code>cond/1</code> &#x8BED;&#x53E5;&#x5982;&#x679C;&#x6CA1;&#x6709;&#x4EFB;&#x4F55;&#x5339;&#x914D;&#x4F1A;&#x629B;&#x51FA;&#x5F02;&#x5E38;&#x3002;&#x6211;&#x4EEC;&#x53EF;&#x4EE5;&#x5B9A;&#x4E49;&#x4E00;&#x4E2A; <code>true</code> &#x7684;&#x6761;&#x4EF6;&#x653E;&#x5230;&#x6700;&#x540E;&#xFF0C;&#x6765;&#x5904;&#x7406;&#x8FD9;&#x79CD;&#x610F;&#x5916;&#xFF1A;</p>
<pre><code class="lang-elixir">iex&gt; <span class="hljs-keyword">cond</span> <span class="hljs-keyword">do</span>
...&gt;   <span class="hljs-number">7</span> + <span class="hljs-number">1</span> == <span class="hljs-number">0</span> -&gt; <span class="hljs-string">&quot;Incorrect&quot;</span>
...&gt;   <span class="hljs-keyword">true</span> -&gt; <span class="hljs-string">&quot;Catch all&quot;</span>
...&gt; <span class="hljs-keyword">end</span>
<span class="hljs-string">&quot;Catch all&quot;</span>
</code></pre>

                                
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